The present invention relates generally to encoders, and more particularly to an optical encoder assembly for an optical encoder for determining rotation of a rotatable shaft.
Encoders include optical encoders which determine rotation of a rotatable shaft by calculating the angular position, angular velocity, and/or angular acceleration of the rotatable shaft. In numerous electromechanical systems, it becomes necessary to precisely determine and control the movement of a driven rotating shaft. Toward this end, optical encoders are often employed. They make use of a disk or codewheel which modulates radiation from an emitter. Detector(s) respond to this modulation by outputting voltage or current, which is used by a control algorithm to change the input to a motor to achieve the desired shaft angular position, angular velocity, or angular acceleration. These optical encoders fall into two broad categories. The first category includes those encoders that are pre-assembled with a shaft section through the body or housing of the encoder and delivered as a complete package for attachment via couplers to the shaft that needs to be controlled. In this case, the alignment between the codewheel, mask, sensors, and shaft has already been set at the vendor""s factory.
The second category of encoders, sometimes referred to as modular encoders, does not have a shaft section built into the body or housing of the encoder, so some form of secondary operation is conventionally required to precisely set the codewheel in relation to the mask and emitter/detector prior to securing the codewheel to the shaft. Modular encoders are typically hand-assembled in place during the fabrication of the rest of the machine that goes with the shaft. Currently, modular optical encoders require additional steps after initial assembly to precisely set the gap between the codewheel, mask (if used), and the emitter/detector. The conventional manner of calibration involves usage of special gauges and instrumentation to iteratively set the codewheel/mask and codewheel/sensor relationship. Another known method eliminates such iteration and involves the usage of a tool to temporarily hold all components in rigid alignment until final fasteners are tightened (U.S. Pat. No. 5,701,007) or uses a linear (U.S. Pat. No. 5,057,684) or a rotating (U.S. Pat. No. 4,794,250) cam that is twisted or plunged, thereby setting the proper mask-to-codewheel and codewheel-to-sensor alignment. In the above three methods, final usage of an auxiliary tool to fasten the proper codewheel to the shaft is required.
What is needed is an optical encoder assembly which during assemblage of its parts automatically sets the proper gaps between the parts without requiring the use of any extra tools.
A first expression of a first embodiment of the invention is for an optical encoder assembly for an optical encoder for determining rotation of a rotatable shaft. The optical encoder assembly includes an encoder housing, a first subassembly, and a second subassembly. The encoder housing is non-engageable with the shaft. The first subassembly includes a receiver plate and an encoder mask. The receiver plate is attached to the encoder housing, has a first side and a substantially opposing second side, and has a through hole and a window both extending from the first side to the second side, wherein the through hole is engageable with the shaft. The encoder mask is attached to the first side of the receiver plate, has a shaft hole engageable with the shaft, and has a mask grating positioned over the window. The second subassembly is attached to the encoder housing and includes a light emitter and a light detector. The light emitter is aligned to face the first side of the receiver plate and is positioned over the mask grating. The light detector is attached to the second side of the receiver plate and is positioned over the window.
In one example, an optical encoder includes the previously-described optical encoder assembly and also includes an encoder codewheel attached to and rotatable with the shaft, radially extending from the shaft to the mask grating, and axially positioned between the light emitter and the mask grating.
Several benefits and advantages are derived from the first expression of a first embodiment of the invention. By having the encoder mask attached to the first side of a receiver plate and the receiver plate attached to the encoder housing, proper positioning of the encoder mask with respect to the housing is assured. By having the light detector attached to the second side of the receiver plate and the receiver plate attached to the encoder housing, proper positioning of the light detector with respect to the housing is assured. In one construction, a socket on the encoder housing surrounds the light emitter and seats on the rim of the light emitter to assure proper positioning of the light emitter with respect to the housing. In the same or another construction which also includes the example having the codewheel, the encoder housing has alignment bumps and the first side of the receiver plate has alignment surface bumps to assure proper positioning of the codewheel which is axially positioned between the alignment bumps and the alignment surface bumps.